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LM5122EVM-2PH: Insufficient output current based on LM5122-Q1 design

Benjamin ZHANG
Prodigy 50 points
Part Number: LM5122EVM-2PH
Other Parts Discussed in Thread: LM5122

Based on the LM5122EVM-2PH development board, I removed the inductors L2 and R21 so that only U1 works. Then replace the output capacitor, Q1, Q2 and loop compensation components C31, R31, C32 and R32, R33, R34. Make the output of the board 48V.
After the test, the board's driving ability is only 1.2A, which can't meet my design requirements. I need 1.5A, so I did the following operation, but I can't increase the output current of the board.
1. Increase L1 and decrease L1
2. Increase output capacitance and reduce output capacitance
3. Increase R2 and decrease R2
What should I do?
I suspect that the drive capability is not enough due to the improper selection of the loop compensation component, but I don't know how to adjust it.

During the debugging process, it was found that the board could start the load normally when it was not connected, and then the load could work normally. However, when the board is started after the load is connected, it cannot be started normally, and the output will be limited to about 32V. What is the reason?

  • 0
    TI__Guru* 78260 points
    Hi Benjamin,

    Thank you for considering the LM5122. I am wondering why you do not use the single phase LM5122EVM for your case. You should also remove Q3 and Q4, and maybe U2 itself. The improperly biased U2 (you removed R21) will have uncertain states.

    Thanks,
    Youhao Xi, Applications Engineering
  • 0 in reply to Youhao Xi
    Prodigy 50 points
    Hello, I follow your suggestion, now the board is working properly, the drive capability can reach 2A, but there is still the second problem mentioned above: the board can start normally without connecting the load, and then connect after booting The load can work normally. However, when the board is started with the load connected, it cannot be started normally, and the output is limited to about 32V.
  • 0 in reply to Benjamin ZHANG
    TI__Guru* 78260 points
    Hi Benjamin,

    What kind of load do you use? An electronic load? In what mode?

    Thanks,
    Youhao
  • 0 in reply to Youhao Xi
    Prodigy 50 points
    The electronic load of chroma is used, the model is 6321A, and it works in CCH mode.
    And I found that for the LM5122EVM-2PH, when the board is started after connecting a load of about 2A, it needs to be powered on to the 14V board to work properly. The higher the load, the greater the voltage required to start the board.
  • 0 in reply to Benjamin ZHANG
    TI__Guru* 78260 points
    Can you change your E-load to CR mode, or try a pure resistor? I am afraid your E-load causes the problem.

    Thanks,
    Youhao Xi, Applications Engineering